Permanent magnet electromagnetic machines (referred to as permanent magnet machines herein) utilize magnetic flux from permanent magnets to convert mechanical energy to electrical energy or vice versa. Various types of permanent magnet machines are known, including axial flux machines, radial flux machines, and transverse flux machines, in which one component rotates about an axis or translates along an axis, either in a single direction or in two directions, e.g. reciprocating, with respect to another component. Such machines typically include windings to carry electric current through coils that interact with the flux from the magnets through relative movement between the magnets and the windings. In a common industrial application arrangement, the permanent magnets are mounted for movement, e.g. on a rotor (or otherwise moving part) and the windings are mounted on a stationary part, such as a stator. Other configurations, typical for low power, inexpensive machines operated from a direct current source where the magnets are stationary and the machine's windings are part of the rotor (energized by a device known as a “commutator” with “brushes”) are clearly also available, but will not be discussed in detail in the following text in the interest of brevity.
In an electric motor, for example, current is applied to the windings in the stator, causing the magnets (and therefore the rotor) to move relative to the windings, thus converting electrical energy into mechanical energy. In a generator, application of an external force to the generator's rotor causes the magnets to move relative to the windings, and the resulting generated voltage causes current to flow through the windings—thus converting mechanical energy into electrical energy.
Surface mounted permanent magnet machines are a class of permanent magnet machines in which the magnets are mounted on a ferromagnetic structure, or backing, commonly referred to as a back iron. Such machines are generally the lowest cost and lightest weight permanent magnet machines, but they typically suffer from limitations in performance that can be traced to the flux density limitations, well known in the art, of conventionally designed and manufactured permanent magnets. As a general matter, flux density can be increased by using magnets formed of a material having a relatively higher magnetic energy density, or of relatively greater thickness. High magnetic energy density materials, such as the neodymium-iron-boron system, are typically more expensive and have historically been subject to significant price volatility. Thicker magnets require more magnetic material, and cost generally scales with the amount of materials. Thus, increasing flux density for such machines with these approaches increases cost and potentially increases cost volatility, and may yield only limited performance improvements. Further, there is an inherent limit to the amount of flux in a given magnetic circuit, where further additions to magnet thickness may yield little to no additional flux.
Given the drawbacks of known techniques of improving the electromagnetic efficiency and other performance attributes of surface mounted machines, new techniques for effecting such performance improvements are clearly desired by those practiced in the art of designing such machines. Further, because many applications of permanent magnets other than permanent magnet machines as described above would benefit from the ability to enhance magnetic performance while limiting cost, such new techniques will be even more desirable if they have broad applicability not limited to permanent magnet machines.
The benefits of the disclosed designs and techniques will be apparent to those practiced in the art of designing and building surface mounted permanent magnet machines. In fact, the benefits of the disclosed designs and techniques may enable surface mounted magnet machines to compete with other permanent magnet machine topologies (such as embedded magnet machines) on performance while retaining the established cost and weight advantages of surface mounted permanent magnet machines. Moreover, the benefits and usefulness of the disclosed designs and techniques are not limited to surface mounted permanent magnet machines, but extend to a wide variety of permanent magnet applications.